The present invention concerns an improved manufacturing method for electronic components including a coil.
Numerous electronic components include a coil formed of a core generally made of ferrite around which is coiled a very fine wire, of a diameter of the order of a few hundredths of a millimeter.
A manufacturing method for such circuits is known from WO Patent No. 91/00603. This method includes a certain number of steps which will be described succinctly here with reference to FIG. 1 annexed to the present Application.
An electronic component, designated as a whole by the general numeric reference 1, includes a ferrite core 2 which is applied against an integrated circuit 4 by means of a mechanical holding device which is not shown in FIG. 1. A distributing device carrying a pay-out reel, onto which is wound a winding wire 6, is brought into position in order to carry out the winding of core 2. The distributing device, moving longitudinally to core 2 and in rotation about the latter, allows contiguous turns 8 of wire 6 to be laid regularly on core 2. After winding a coil 10 onto core 2, wire 6 is cut and the distributing device is moved in front of the next ferrite core. The free ends 12 and 14 of wire 6 wound onto core 2 are then taken up by a suitable device and applied onto contact bumps 16 of integrated circuit 4.
Electronic components having a coil of the type described hereinbefore are used in particular in the horological industry and the automobile industry where they are used in association with vehicle coded anti-starting systems. In this latter case, integrated circuit 4 and the coil 10 associated therewith which forms a reception antenna are integrated in a tube 18 made of glass or a similar material as shown in FIG. 2 annexed to the present Application. Tube 18 is initially open at one of its ends, and contains a thermosetting epoxy resin 20. After a centrifugation step which allows electronic component 1 to be moved to the bottom of tube 18, the latter is conventionally sealed by means of a high power laser beam. The last step of the method consists in polymerising resin 20 by placing sealed tube 18 in a furnace. Tube 18 is then moulded for example into the thickness of a handle for an automobile switch key. Integrated circuit 4 enclosed in tube 18 includes an identification code which is transmitted by radio-frequency link to the central electronic control unit of the vehicle. After receiving the identification code and checking its authenticity, the central unit orders the unlocking of the vehicle anti-starting system.
The above method has advantageously allowed the manufacture of electronic components including a coil to be mechanised and automated, thus contributing to a significant drop in the cost price of the finished coils. It has nonetheless been noted that this new manufacturing method was accompanied by significant problems of electrostatic discharge. Turns 8 of wire 6 are wound onto core 2 at great speed, typically of the order of 50,000 revs per minute. During winding of wire 6, turns 8 of coil 10 rub against each other, which leads to the appearance of electrostatic charges at the surface of wire 6. Since the outer covering of wire 6 is electrically insulated by means of an insulating lacquer in order to avoid creating short circuits between contiguous turns 8, the electrostatic charges accumulate in coil 10 and cannot be eliminated. In the last step of the method when free ends 12 and 14 of coil 10 are applied onto contact bumps 16 of integrated circuit 4, the static electricity is discharged abruptly into integrated circuit 4 and destroys it. Observed losses due to electrostatic discharge are currently of the order of 20% to 30% of integrated circuits 4.
A manufacturing method for coils for transformers in which a winding wire wound onto a pay-out reel is placed in turns on a reel is also known from Japanese Patent No. 56 108663. Prior to winding the wire onto the reel, the latter is cleaned by means of a flexible brush device, then circulated in a cylindrical conduit into which ionised air is injected for the purpose of cancelling out the static electricity charges which appear in said wire during brushing. This document which can be considered the state of the art closest to the present invention thus suggests cancelling out the electrostatic charges prior to winding the wire onto the reel, and does not raise aforementioned problem of electrostatic discharges. This difference with the present invention is explained by the fact that, in the Japanese document, the electrostatic charges appear during brushing, and it is sought to remove these charges immediately after brushing and prior to winding. Moreover, the winding speeds in question within the scope of the present invention and in the Japanese document are very substantially different. Indeed, in the case of the present invention, the winding speed is very high (of the order of 50,000 revs per minute), hence significant friction between the turns which generated a large number of electrostatic charges which it is sought to remove. Conversely, the Japanese Patent concerns coils for transformers whose dimensions are significant, and for which the winding speed is lower, so that the winding of the wire is not accompanied by the creation of electrostatic charges. Consequently, the Japanese Patent suggests at most installing an air ionising device upstream of the enclosure in which the coil is manufactured, and does neither provide nor suggest any solution to the problem solved by the present invention.
Japanese Patent No. 57 091235, which proposes arranging an air ionising device at the centre of a confined manufacturing enclosure, is also known. This document concerns the manufacture of plastic tapes which is a distant technical field not forming part of the knowledge of those skilled in the art of the invention who, in the present case, are electronic component manufacturers. Further, in this Japanese document, it is sought simply to cancel out the electrostatic charges which appear in the plastic tape on exiting the extruder, during the passage of the tape between the shaping and driving rollers, in order to avoid attracting dust which could become incorporated in the still hot plastic material. The problem raised by the invention, namely to seek to remove the electrostatic charges which are generated during winding of a wire at a very high speed, is not described, nor even suggested.